In such variators, as is well known, grouped rollers of variable orientation transmit traction between matching part-toroidal races formed on the confronting faces of coaxial and contra-rotating input and output discs. The rotary velocity transmitted from the input to the output disc changes when the orientation of the rollers changes: when the rollers contact the input disc at a relatively high radius and the output disc at a relatively low one, the output disc rotates faster than the input disc and the variator is said to be set at a high ratio, whereas if the rollers contact the input disc at low radius and the output disc at high radius then the output disc rotates slower and the variator is in low ratio. Such variators have been described in many prior patent specifications within this century, for instance U.S. Pat. No. 1,056,292, GB-A-1078791, and more recently WO90/05860. More particularly, the invention applies to variators of the toroidal-race rolling-traction type having two further characteristics. Firstly, the rollers span the part-toroidal clearance between their respective input and output discs substantially diametrically, so that the centre of each roller lies substantially on the imaginary centre circle of the torus. The invention thus applies to variators of the so-called "full toroidal" type of which all three of the prior patent specifications Just recited show examples, but not to variators of the so-called "half toroidal" type, which pose quite different problems of construction and operation and of which specification U.S. Pat. No. 4,400,987 shows one example. Secondly, the invention applies to variators of the full-toroidal type in which, as in specification WO90/05860, the centre of each roller must be free in normal operation of the variator, for instance during ratio changes, to make limited movements back and forth around the circumference of the torus centre circle. The invention does not apply to the alternative type of toroidal-race variator, of which specification GB-A-1069874 shows examples, in which the roller centre is constrained to a single location on the torus centre circle.
Diagrammatic FIGS. 1 and 2 of the accompanying drawings show certain features and components of a variator of the type to which the invention can apply. FIG. 1 is a view in a plane at right angles to the disc axis and FIG. 2 a view in a plane including that axis. In FIG. 1 a roller A ia mounted to rotate about its centre B which mounted in a carriage C connected to a piston D working in a cylinder E. Roller A transmits traction from a part-toroidal race F formed in a disc G to a corresponding race H formed on a coaxial disc J. As in examples shown in specification WO90/05860, piston D not only moves axially within cylinder E but can also tilt slightly relative to the cylinder axis. Both Figures show clearly that any control movement of roller A, by movement of piston D within cylinder E, must have the effect of moving the centre B of the roller. As FIG. 2 shows best, because roller A spans the toroidal clearance diametrically, the consequent movement of centre B must be along the centre circle K of the torus. FIG. 1 also shows that the line of action of piston D on roller A lies at an angle (L) to the plane M which includes the centre circle K. The angle L is known in the art as caster angle, and is present in many known variators of this kind including the ones described in specification WO90/05860.
It is a well known characteristic of variators of toroidal-race CVT's, in which such caster angle is present, that they are unidirectional in operation. That is to say, predictable changing of the transmitted ratio, by unison variation of the orientation of the rollers, is only possible if the input disc or discs are rotating in one rotary sense about the variator axis, and the output disc or discs in the other sense. The reason for this is well understood in the art, and in summary is that the necessary geometry for the mounting of the rollers includes caster angle as shown in FIG. 1. It is therefore important that the input shafts of variators of CVT's of this type should never rotate backwards to any material degree.
In practice the internal combustion engines that have usually served as the prime movers for variators of this type have been designed to rotate forwards only, and the slight backwards movements that they have been subject to, for instance to take up an equilibrium position when shut down, have been insufficient to upset the roller orientation of previous generations of toroidal-race variators, because no more than say a single backwards revolution ever took place at one time. The response rate of such variators required several revolutions of the input disc or discs for the rollers to steer themselves through any substantial change of orientation. The present invention arises from the fact that with a more modern hydraulic mounting and control of the rollers, as described in WO90/05860 for instance, substantial change of roller orientation is much more rapid, and can occur within as little as a small fraction of a revolution of the input disc or discs. The possibility of the rollers changing orientation rapidly and unpredictably, and even running off the input and/or output races, within a single backward revolution of the input shaft, for instance on engine shut down or following a false start, therefore becomes a real one and should be guarded against.